Original Research
published: 26 April 2017
doi: 10.3389/fneur.2017.00167
S
Vibeke M. Bruinenberg1, Marijke C. M. Gordijn2,3, Anita MacDonald 4,
Francjan J. van Spronsen 5 and Eddy A. Van der Zee1*
Molecular Neurobiology, Groningen Institute for Evolutionary Life Sciences (GELIFES), University of Groningen, Groningen,
Netherlands, 2 Chrono@work B.V., Groningen, Netherlands, 3 Chronobiology, Groningen Institute for Evolutionary Life
Sciences (GELIFES), University of Groningen, Groningen, Netherlands, 4 Birmingham Children’s Hospital, Birmingham, UK,
5
Beatrix Children’s Hospital, University Medical Center Groningen, Groningen, Netherlands
1
Edited by:
Birendra N. Mallick,
Jawaharlal Nehru University, India
Reviewed by:
Liborio Parrino,
University of Parma, Italy
Axel Steiger,
Max Planck Institute of
Psychiatry, Germany
*Correspondence:
Eddy A. Van der Zee
[email protected]
Specialty section:
This article was submitted
to Sleep and Chronobiology,
a section of the journal
Frontiers in Neurology
Received: 10 February 2017
Accepted: 07 April 2017
Published: 26 April 2017
Citation:
Bruinenberg VM, Gordijn MCM,
MacDonald A, van Spronsen FJ and
Van der Zee EA (2017) Sleep
Disturbances in Phenylketonuria: An
Explorative Study in Men and Mice.
Front. Neurol. 8:167.
doi: 10.3389/fneur.2017.00167
Frontiers in Neurology | www.frontiersin.org
Sleep problems have not been directly reported in phenylketonuria (PKU). In PKU,
the metabolic pathway of phenylalanine is disrupted, which, among others, causes
deficits in the neurotransmitters and sleep modulators dopamine, norepinephrine,
and serotonin. Understanding sleep problems in PKU patients may help explain the
pathophysiology of brain dysfunction in PKU patients. In this explorative study, we
investigated possible sleep problems in adult treated PKU patients and untreated PKU
mice. In the PKU patients, sleep characteristics were compared to healthy first degree
relatives by assessment of sleep disturbances, sleep–wake patterns, and sleepiness
with the help of four questionnaires: Holland sleep disorder questionnaire, Pittsburgh
sleep quality index, Epworth sleepiness scale, and Munich Chronotype Questionnaire.
The results obtained with the questionnaires show that PKU individuals suffer more
from sleep disorders, a reduced sleep quality, and an increased latency to fall asleep
and experience more sleepiness during the day. In the PKU mice, activity patterns
were recorded with passive infrared recorders. PKU mice switched more often
between active and non-active behavior and shifted a part of their resting behavior
into the active period, confirming that sleep quality is affected as a consequence of
PKU. Together, these results give the first indication that sleep problems are present in
PKU. More detailed future research will give a better understanding of these problems,
which could ultimately result in the improvement of treatment strategies by including
sleep quality as an additional treatment target.
Keywords: inherited metabolic disorder, Pahenu2 mice, phenylketonuria patients, phenylketonuria mice,
neurotransmitters, sleep disorders
INTRODUCTION
In 2014, Gadoth and Oksenberg reviewed the incidence of sleep and sleep disorders in patients
with inherited metabolic disease (IMD). Although their review focused on sleep-related breathing
disorders among severely affected subjects with IMD, the general title suggested that sleep problems
could be missed or underestimated in these conditions. A metabolic disease in which brain modulators of sleep are severely affected but attention for sleep research is very limited is phenylketonuria
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Bruinenberg et al.
Sleep Disturbances in PKU
(PKU). PKU is caused by an inborn error in the metabolic
pathway of phenylalanine (Phe) that disrupts the conversion of
Phe to tyrosine. As a result, Phe concentrations build up in blood
and brain and the ability to intrinsically produce the dopamine
precursor tyrosine is lost. These changes do not solely affect
the metabolism of dopamine. Also, reduced concentrations of
noradrenaline and serotonin are found in PKU patients (1, 2) and
in the PKU mouse model (3). These neurotransmitters are known
to be important regulators of sleep, wakefulness, and switches
between these states (4, 5). Nevertheless, these abnormalities in
neurotransmitter availability are not specifically linked to possible sleep problems in PKU research. In PKU research, a few
studies have indirectly investigated sleep regulators or sleep. First,
in treated and untreated PKU patients, sleep-EEG measurements
indicate differences in the number of sleep spindles despite
similar REM and non-REM distribution compared to healthy
controls (6). Second, in early-treated PKU infants (4–18 weeks
old), EEG measurements show differences in the development of
sleep compared to healthy controls (7). Finally, in the PKU mouse
model, high levels of orexin A (hypocretin 1) were reported, a
neuropeptide that is associated with wakefulness (8, 9). This
made the authors suggest hyperactivity in PKU, however, the
exact consequence of these increased levels are not clear while
hyperactivity is not consistently described in PKU mice (10).
Currently, PKU treatment remains suboptimal in which
disturbances in executive functions, mood, social cognition,
and in internalizing problems, such as depression and anxiety,
are described in early-treated PKU patients (11, 12). As it is well
established that altered sleep negatively influences cognitive performance, most notably in the domains of executive functioning
(13–15), and mood by impacting feelings of depression, anxiety,
and stress (16, 17), sleep-related issues could very well serve as
an explanation of the PKU brain dysfunction despite diet and
drug treatment.
Understanding the presence and severity of sleep problems
in PKU patients and its pathophysiology could ultimately result
in the improvement of treatment strategies by including sleep
quality as an additional treatment target. Therefore, the aim of
this explorative study was to investigate the presence of sleep
disturbances in PKU patients with questionnaires together with
analyses of rest/wake patterns in PKU mice, indirectly reflecting
sleep characteristics which could confirm the PKU-specific nature
of putative sleep issues in PKU patients. As sleep is influenced
by, among others, genetic factors (18–21), first-degree relatives
(FDR) of PKU patients and wild-type (WT) littermates of each
genetic strain of the PKU mouse model were used as controls.
of birth, zip code, gender, height, bodyweight, PKU or control,
treatment of PKU, other health issues, smoking, and the use of
sleep-promoting drugs). All participants were informed about
the scientific purpose of the study and agreed to participate. They
completed the questionnaires completely anonymous. To ensure
that the questionnaires were not completed by the same individuals more than once, the submissions were checked for uniqueness,
focusing on date of birth and IP address. In total, 47 subjects,
25 PKU patients and 23 controls, participated.
Sleep Questionnaires
Four validated questionnaires were included in the survey: (1)
Holland Sleep Disorders Questionnaire (HSDQ) (22); 40 items,
(2) Pittsburgh Sleep Quality Index (PSQI) (23); 19 items, (3)
Epworth Sleepiness Questionnaire (ESS) (24); 8 items, and (4)
Munich Chronotype Questionnaire (MCTQ) (25); 16 items.
First, the HSDQ gives a general score to identify the possible
occurrence of a sleep disorder and can differentiate between six
main categories of sleep disorders [insomnia, parasomnia, circadian rhythm sleep disorders (CRSD), hypersomnia, sleep-related
movement disorders, such as for instance restless legs syndrome,
and sleep-related breathing disorder] (22). Second, for the PSQI,
seven component scores can be derived (subjective sleep quality, sleep latency, sleep duration, habitual sleep efficiency, sleep
disturbances, use of sleep medication, and daytime dysfunction)
and computed to a global score (23). Third, the ESS is used to
examine sleepiness during the day (for instance during reading)
(24). Finally, the MCTQ was used to identify chronotype (the
preferred timing of sleep) calculated by taking the mid-point of
sleep on free days corrected for the sleep debt acquired during
working days (26).
PKU Mouse Study
To investigate the rest/wake pattern in PKU mice, the home-cage
activity of adult (4–7 months) WT and PKU mice of BTBR and
C57Bl/6 (B6) background was monitored. Both genetic strains of
the PKU mouse model have a point mutation in the gene encoding for phenylalanine hydroxylase causing Phe to rise in blood
and brain (10). The PKU model was originally described for the
BTBR background but the model was latter crossed back on to
the C57Bl/6J background. Currently, both genetic strains are used
in PKU research. In-house heterozygous mating pairs were used
to breed the following groups of mice: BTBT WT, BTBR PKU,
B6 WT, and B6 PKU. These mice were weaned on postnatal day
28, and genotype was established with quantitative PCR (10). The
experiment consisted of a habituation phase and a data-acquiring
phase. After a 7-day habituation phase, the activity of individual
housed mice (cage: 33 cm × 15 cm × 14 cm with nesting material
and paper role) were monitored for 7 days with passive infrared
detectors (PIR). During the whole experiment, animals were
on a 12/12 light/dark cycle and had ad libitum access to water
and normal chow (RMH-B 2181, ABdiets, Phe: 8.7 g/kg). Data
were analyzed with ACTOVIEW [made in-house, described in
Ref. (27)]. This program calculated the average daily activity, diurnality [(Sum activity light phase − sum activity dark phase)/total
activity], and fragmentation (28) from the files produced by the
circadian activity monitor system (CAMS) collected from the PIR.
MATERIALS AND METHODS
PKU Patients
Subjects
In the summer of 2016, participants for this study were recruited
by distributing a link to an electronic survey to subjects associated with the Dutch PKU patient organization. PKU patients and
FDR, who did not do shift work in the past 3 months, were asked
to fill out four questionnaires with 10 additional questions (date
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Statistics
Frequency of Sleep Disorders
The statistical analysis was executed with the statistical software
IBM SPSS Statistics for Windows, Version 22.0 (Armonk, NY,
USA: IBM Corp.). Within this program, the Shapiro–Wilk was
used to test normality of the data. The activity of the mice was
normally distributed, and a multi-variate ANOVA was used to
test the factors group and gender of overall activity, fragmentation, and diurnality. Differences in group characteristics in the
patient study were not normally distributed and, therefore, tested
with the non-parametric Mann–Whitney U test. The chronotype
score of the MCTQ, normally distributed, was tested with a
univariate ANOVA for group. Age and gender were included as
cofactors. The ordinal nature of the HSDQ, PSQI, and ESS scores,
made it not possible to test confounding factors parametrically.
Therefore, gender and age were explored with generalized linear
models, using an ordinal model. Differences in the frequency of
occurrence of sleep disorders were also tested with a generalized
linear model, using a binary model. Non-parametric Levene’s test
was used to test the homogeneity of the data.
The global score of the HSDQ is used to identify the presence of
a sleep disorder with an overall accuracy of 88% (κ: 0.75) (22).
In PKU patients, 48% had a global score above the cutoff of 2.02,
indicative of a sleep disorder, compared to 19% of FDR controls
[Figure 1A; b = 1.367, Wald χ2(1, N = 46) = 3.983, p < 0.05].
Especially, among the six main sleep disorder categories, PKU
patients had a higher score for both insomnia and CRSD
[Figure 1B: b = −1.527, Wald χ2(1, N = 46) = 7.626, p < 0.05,
Figure 1C: b = −1.593, Wald χ2(1, N = 46) = 8.115, p < 0.05,
respectively], but not for the other four categories [parasomnia;
b = −0.985, Wald χ2(1, N = 46) = 2.941, p = 0.086, hypersomnia;
b = −0.985, Wald χ2(1, N = 46) = 3.287, p = 0.070, restless legs
syndrome; b = −0.696, Wald χ2(1, N = 46) = 1.757, p = 0.185,
and sleep-related breathing disorder; b = −0.693, Wald
χ2(1, N = 46) = 1.688, p = 0.194]. Age and gender did not significantly contribute to these models. These results reveal a higher
frequency of sleep disorders, more specifically insomnia and
CRSD, in PKU patients.
RESULTS
Sleep Quality
The global PSQI, comprised seven sleep-related components, is
used to classify poor (>5) and good sleepers (≤5). This global
score was significantly higher in the PKU patients of which more
people were classified as poor sleepers (57%) compared to the
FDR controls of which 25% were classified as poor sleepers
[Figure 2A: b = −1.674, Wald χ2(1, N = 37) = 7.102, p < 0.05].
Within the global score, two component scores differed between
PKU patients and FDR controls. The first was the component
score for latency to fall asleep which was significantly increased in
PKU patients [FDR: 0.44 ± 0.63, PKU: 1.43 ± 0.98, b = −2.306,
Wald χ2(1, N = 37) = 9.733, p < 0.05]. The second one was the
component score for subjective sleep quality. This score was computed from the question in which subjects could indicate their
quality of sleep during the past month on a scale of very good
(score 0) to very bad (score 3). This score was significantly higher
in PKU patients than in the FDR controls [FDR: 0.69 ± 0.60,
PKU: 1.38 ± 0.92, b = −1.655, Wald χ2(1, N = 37) = 5.516,
p < 0.05]. Age and gender did not significantly contribute to
PKU Patients
Subjects
This study is a pilot study serving as a proof-of-concept for sleeprelated issues due to PKU. For this reason, individuals were included
when they correctly filled out at least one of the questionnaires. Not
all questionnaires were correctly filled out by all subjects, which
resulted in different group characteristics for each questionnaire
(Table 1). For all questionnaire responses, the FDR controls were
significantly older than the PKU subjects [HSDQ: p < 0.05, PSQI:
p < 0.05, Epworth Sleepiness Questionnaire (ESS): p < 0.05, MCTQ:
p < 0.05] but no differences were found in BMI (HSDQ: p = 0.68,
PSQI: p = 0.92, ESS: p = 0.99, MCTQ: p = 0.90). Furthermore, no
significant differences were found in gender distribution of the
groups [HSDQ: t(44) = −0.603, p = 0.55, PSQI: t(35) = 0.172,
p = 0.87, ESS: t(37) = 0.143, p = 0.89, MCTQ: t(39) = −0.260,
p = 0.80]. One FDR control was excluded because she used several
sleep-promoting drugs (Trazodon and zolpidem tartrate).
Table 1 | Subject characteristics.
HSDQ
Number
Age
BMI
Gender (F/M)
Smoking
Heath issues
Sleep-promoting drugs
Kuvan®
Kuvan® + protein restricted
Protein restricted
PSQI
ESS
MCTQ
PKU
FDR-control
PKU
FDR-control
PKU
FDR-control
PKU
FDR-control
25
29.2 ± 8.8
24.1 ± 4.3
17/8
4
5
5
3
4
22
21
44.2 ± 10.1
25.0 ± 5.1
16/5
1
0
0
21
30.1 ± 9.0
24.5 ± 4.3
15/6
4
4
5
2
3
16
15
46.6 ± 10
24.8 ± 5.6
11/4
1
0
0
22
29.6 ± 9.1
24.6 ± 4.2
16/6
4
4
5
2
3
17
17
45.9 ± 10.1
24.6 ± 4.2
12/5
1
0
0
24
29.0 ± 8.9
24.4 ± 4.1
16/8
4
5
5
3
3
18
17
45.9 ± 10.1
24.6 ± 4.2
12/5
1
0
0
Data are presented as mean ± SD.
F, female; M, male; HSDQ, Holland Sleep Disorders Questionnaire; PSQI, Pittsburgh Sleep Quality Index; ESS, Epworth Sleepiness Questionnaire; MCTQ, Munich Chronotype
Questionnaire; PKU, phenylketonuria; FDR-control, first-degree relatives-control.
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Chronotype
In the MCTQ, the sleep schedules of the participants on working
and non-working days were asked. From these data, we could
calculate chronotype, defined as the mid-sleep on free days corrected for the potential sleep debt acquired during the working
days (26). Chronotype is dependent on age and gender (26),
therefore, statistical analysis included age and gender as a cofactor.
No significant differences were found between chronotype scores
in PKU patients and FDR controls or for any of the cofactors in
the complete model [group: F(1,37) = 1.287, p = 0.26, gender:
F(1,37) = 0.954, p = 0.34, age: F(1,37) = 1.941, p = 0.17].
PKU Mice
Rest/Wake Patterns
No main or interaction effects were observed for sex in all parameters, therefore, data of males and females were grouped. The fragmentation score is indicative of the frequency that active behavior
is switched to non-active behavior and vice versa. In PKU mice,
in both strains, the fragmentation score was increased compared
to WT [Figure 3A; F(3,56) = 10.803, p < 0.05, BTBR p < 0.05,
p < 0.05]. Although differences were found in overall activity
between the WT’s of each strain [BTBR WT: 3,162.46 ± 1,239.24,
B6 WT: 2,193.69 ± 785.91 (mean ± SD) F(3,56) = 3.858, p < 0.05,
BTBR WT vs. B6 WT p = 0.019], the increase in fragmentation
score found in PKU mice did not coincide with a change in overall
activity (BTBR PKU: 2,655.42 ± 605.23, B6 PKU: 2,306.90 ± 860.06,
BTBR p = 0.67, B6 p = 1.000). However, a shift did occur in the
timing of the rest/active behavior. The negative diurnality score,
reflecting night activity in animals, became less negative in PKU
mice [Figure 3B; F(3,56) = 8.235, p < 0.001, BTBR p < 0.05, B6
p < 0.05]. These results reveal that PKU mice have increased fragmentation and a shift in diurnality (more inactive in active phase).
DISCUSSION
Figure 1 | Holland Sleep Disorders Questionnaire (HSDQ). (A) Results
from the global score of HSDQ indicate that 48% of the phenylketonuria
(PKU) patients have a sleep disorder compared to 19% of the first-degree
relatives (FDR) controls. (B) PKU patients have a significant higher insomnia
score than FDR controls. Six PKU patients are above the cutoff score
compared to 0 FDR controls. (C) Although only two PKU patients are above
the cutoff score, PKU patients have significant higher circadian rhythm sleep
disorders score compared to FDR controls. Data represent individual scores
with median. Dotted line represents cutoff score between having sleep
problem or not (**p < 0.01).
In this explorative study, we investigated sleep characteristics in
PKU patients with questionnaires and analyzed the rest/wake
patterns in PKU mice. In the PKU patients study, we showed
that PKU patients compared to FDR controls have more sleep
disorders, a reduced sleep quality, an increased latency to fall
asleep, and experience more sleepiness during the day. In the
PKU mice, we found an increase in fragmentation and a shift in
diurnality. The increase in fragmentation indicated that the PKU
mice switch more often between active and non-active behavior.
This score did not coincide with changes in overall activity. In
addition, PKU mice shift a part from their resting behavior into
the active phase (a shift in diurnality). Both experiments strongly
support the hypothesis that sleep is affected in PKU. This seems
to be directly related to the disorders as the deficits were found
in both PKU mouse strains despite their genetic differences and
cognitive sensitivity to the PKU condition (10).
these models. These results suggest that sleep quality is reduced
in PKU patients compared to FDR controls.
Sleepiness during the Day
In the ESS, the participant subjectively rate the chance of dozing
off during eight situations from “none” (score 0) to “high” (score
3). This score is significantly higher in PKU patients than FDR
controls [Figure 2B; b = −1.608, Wald χ2(1, N = 39) = 6.597,
p < 0.05]. A main effect of age and gender did not contribute
significantly to the model. These results suggest that PKU patients
experience more sleepiness during daytime.
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Study Limitations
Sleep is influenced by, among others, genetic factors, age, and
gender (18–21). For this reason, this study recruited FDR of PKU
patients as control group. No differences were found in gender
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Figure 2 | Pittsburgh Sleep Quality Index (PSQI) and ESS. (A) The global score of the PSQI was significantly higher in phenylketonuria (PKU) patients
compared to first-degree relatives (FDR) control. The dotted line represents the cutoff between good and poor sleepers. Fifty-seven percent of the PKU patients
are above this cutoff and categorized as poor sleepers. (B) Epworth Sleepiness Questionnaire (ESS) patients experience more sleepiness during the day than FDR
controls. Data represent individual scores with median (*p < 0.05, **p < 0.01).
Figure 3 | Characteristics of the rest/wake pattern in mice. (A) In both genetic strains of the phenylketonuria (PKU) mouse model, an increase in
fragmentation is seen in the PKU mice compared to WT littermates. Furthermore, a significant difference is found between the fragmentation score of PKU mice of
each strain. (B) In the graph, negative diurnality scores are observed. This indicates that we are investigating animals which are active in the dark. PKU mice have a
less distinct negative score suggesting that they shift part of their resting behavior into the light phase. Data are depicted as mean ± SEM (*p < 0.05, **p < 0.01).
distributions between the groups, but differences were found in
age between FDR controls and PKU patients. Although we recognize these limitations, we believe that it does not hamper our
results obtained in the first three questionnaires (HSDQ, PSQI,
ESS) for the following reason. Literature shows either no effect or
a deterioration of sleep with aging. For example, the PSQI is not
affected by age (29). The ESS score is influenced by an age × gender
interaction, wherein females tend to have higher scores between 0
and 39 age group compared to males (30). Around the ages 40–49,
the ESS score of males deteriorates reaching the same ESS score
as females. Therefore, the ESS score is either higher or stays constant with increasing age (30). This implies that the higher scores
found in the younger PKU patients compared to the older FDR
controls are contrary to what would be expected and likely reflect
a real indication of sleep problems in PKU patients. Moreover, no
significant effect of age was found in our study.
incidence in sleep disorders, but only in the main categories
insomnia and CRSD scores were higher in PKU patients than in
FDR controls. CRSD were changed to circadian rhythm sleep–
wake disorders (CRSWD) in the third edition of the International
classification of sleep disorders (31). CRSWD are sleep disorders
grouped under dyssomnias, a group of sleep disorders, which
show insomnia, excessive sleepiness, or difficulty initiating or
maintaining sleep (31). In the current study, we found an increased
score for insomnia and an increased sleepiness during the day in
the ESS in PKU patients, supporting the idea that PKU patients
experience problems specific for this cluster of sleep disorders.
CRSWD are disorders related to the timing of sleep (31). Some
are a consequence of external circumstances, e.g., shift work or jet
lag, others have potentially a more internal, neurological basis, e.g.,
delayed sleep phase syndrome (DSPS). In DSPS, the latency to fall
asleep opposed to the desired time to fall asleep is delayed. DSPS
patients experience difficulties to shift their sleep/wake pattern
to an earlier time point in response to environmental time cues,
for example traveling from Europe to Asia, and do not experience
sleepiness when they are able to sleep at their desired time, as is
possible for instance during a holiday or vacation period. In this
Sleep Characteristics Are Altered in PKU
The different measurements of sleep used in this study reveal a
variety of sleep problems and show some specifically affected
characteristics of sleep. In the HSDQ, PKU patients show a higher
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Conclusion
study, we showed that PKU patients report an increased latency
to fall asleep in the PSQI. However, we did not see a difference
in the mid-sleep on non-working days when age was a cofactor.
This could be because mid-sleep is strongly influenced by age and
the age was not distributed evenly over the full width of both
groups (26). Therefore, an important future direction is to compare age-matched controls to PKU patients. Further research in
PKU should focus on (1) more objective measurements of sleep,
such as polysomnography or sleep–wake rhythm analysis with
(2) phase shift experiments in PKU mice to identify problems
in shifting sleep/wake patterns (and neurological substrates), (3)
monitoring sleepiness during the day specifically during holidays
and (4) core body temperature and dim-light melatonin rhythm
monitoring to investigate if PKU patients experience a blunted or
delayed internal rhythm of physiological markers.
This explorative study is the first to investigate sleep disturbances
both in PKU patients and PKU mice. In PKU patients, we demonstrate more sleep disorders, a reduced sleep quality, an increased
latency to fall asleep, and more sleepiness during the day. We
show in PKU mice an increased fragmentation and a shift in
diurnality. These results produce the first evidence to suggest that
sleep problems occur in PKU. The resulting complaints associated with altered sleep are comparable to the cognitive symptoms
described for early and continuously treated PKU patients. More
detailed future research will give a better understanding and
further identify sleep problems in PKU, which could ultimately
result in the improvement of treatment strategies by including
sleep quality as an additional treatment target.
ETHICS STATEMENT
The Switch between Sleep and
Wakefulness Is Defective in PKU
The Medical Ethics Review Committee of the University of
Groningen concluded that the Medical Research Involving
Subjects Act did not apply to this study. All proceedings concerning the mouse study were carried out in accordance to
the Guide for the Care and Use of Laboratory Animals of the
National Institutes of Health (The ARRIVE Guidelines Checklist)
approved by the Institutional Animal Care and Use Committee of
the University of Groningen.
The HSDQ identifies sleep disorders and attribute certain scores
to six symptom clusters. These clusters may be due to different sleep disorders, possibly with different pathophysiological
mechanism. For instance, the comorbidity of insomnia with
other sleep disorders is very high (22). The PKU mouse study
did identify a more specific aspect of sleep, namely increased
fragmentation. In general, an increased fragmentation score
indicates an increase in switching behavior. Switching between
sleep and wakefulness is thought to be regulated by a flip-flop
switch that results from mutual inhibition of sleep-promoting
pathways and wake-promoting pathways (32). Several cholinergic and monoaminergic projections are important in these
pathways, such as serotonin, norepinephrine, and dopamine
(32). As these latter neurotransmitters are affected in PKU mice
(and in untreated PKU patients), it could be that the increased
fragmentation score is a consequence of disruptions in this
switch. In early-treated patients on diet, neurotransmitter deficiencies in dopamine and serotonin are still present (2). These
deficiencies could possibly affect the switch between sleep and
wakefulness and cause fragmentation of the sleep/wake rhythm
in PKU patients, more difficulty falling asleep and as a consequence daytime sleepiness.
AUTHOR CONTRIBUTIONS
For the patient study, all authors designed the study. MG made
the electronic survey. Data analysis was performed by MG and
VB. For the mouse study, EZ and VB designed the study. VB
performed the study and the analysis of the data. All authors
contributed substantially to the interpretation of the data and
drafting the manuscript.
ACKNOWLEDGMENTS
The authors would like to thank Dr. Marina C. Giménez for her
help with the electronic survey.
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Conflict of Interest Statement: VB has received grant funding from Nutricia
MG is CEO of Chrono@work. A company specialized in sleep research. In
addition, this author has received research funding from Philips. AM has
received research funding and honoraria from Nutricia, Vitaflo International,
Merck Serono, chaired/chairs the European Nutrition Expert Panel (Merck
Serono international and later Biomarin), was/is a member of the Sapropterin
Advisory Board (Merck Serono international, Biomarin), and is a member of
the Advisory Boards Element (Danone-Nutricia) and Arla Foods International.
FS is a member of various Scientific Advisory Boards of (in past Merck Serono,)
Biomarin, Applied Pharma Research, ARLA Food, and Danone and has received
grants for research purpose of both Merck Serono, Biomarin, Sobi and Nutricia.
Furthermore, he has received consultation fees and speaker relation fees from
Merck Serono, Biomarin, Nutricia, and Vitaflo. He has assisted in the design
of clinical studies using products manufactured by Biomarin. He chairs the
Scientific Advisory Board of the Dutch and the European PKU society and is
the chairman of the group developing guidelines on PKU in Europe. EZ has
received grant funding from Nutricia and awards from NPKUA All authors
confirm independence from the sponsors; the content of the article has not been
influenced by the sponsors. The remaining authors declare that the research was
conducted in the absence of any commercial or financial relationships that could
be construed as a potential conflict of interest.
Copyright © 2017 Bruinenberg, Gordijn, MacDonald, van Spronsen and Van der Zee.
This is an open-access article distributed under the terms of the Creative Commons
Attribution License (CC BY). The use, distribution or reproduction in other forums
is permitted, provided the original author(s) or licensor are credited and that the
original publication in this journal is cited, in accordance with accepted academic
practice. No use, distribution or reproduction is permitted which does not comply
with these terms.
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